* plugin/cancel: add context cancelation plugin

Per review comments on #2704, move this into a plugin that gets called.
Add the most minimal plugin, tests and documenation.
Signed-off-by: Miek Gieben <miek@miek.nl>

* plugin/cache: add timeout option

review feedback: add option to set custom timeout.
Signed-off-by: Miek Gieben <miek@miek.nl>

* spelling
Signed-off-by: Miek Gieben <miek@miek.nl>